Research and development efforts on integrated power supplies that include DC-DC converters are improving and making them more adaptable to different applications. These power supplies are high performance in terms of response to load transients and provide a broad stability range. In addition, these power supplies provide excellent static and dynamic characteristics for reducing electromagnetic radiation. It is desirable to reduce or minimize the number of circuit components needed for these power supplies, particularly when included in VCRs, TVs, receivers, satellite decoders and other similar consumer products. Consequently, the cost of these items can be reduced.
In recent years, many specially designed devices have been produced and marketed to serve these economically important applications. Leading manufacturers of electronic components, such as STMicroelectronics, the assignee of the present invention, market a wide range of monolithic and hybrid devices designed to handle output powers in a range of about 30 to 300 W.
Strict requirements, such as performance specifications, often require a customized product to simplify layouts and to reduce the number of components necessary to form the power supply. A customized product also reduces production costs. The configuration most frequently used in a broad range of consumer products is a flyback self-oscillating power supply (SOPS). A typical flyback scheme is shown in FIG. 1. This basic scheme of a DC-DC converter circuit provides control for transferring power having a switching frequency of about 100 kHz. A relatively small flyback transformer is used for providing high efficiency in transferring power from the primary circuit to the secondary or output circuit.
Generally, a variable frequency control is implemented in a SOPS converter. The conduction phase (T.sub.ON) of the power switch that drives the primary winding is limited, as in a PWM system, by the maximum current and by the output voltage error. The primary winding stores the phase of energy in the flyback transformer. The duration of the nonconducting phase (T.sub.OFF) is determined by the applied load. The non-conduction phase is the stored energy that is transferred to the load via the output of the power supply. In support of the variable frequency control, the zero-crossing of the voltage induced by the current flowing in the secondary winding on a third or auxiliary winding is detected. Consequently, a logic signal for acknowledging the occurrence of such a zero-crossing turns on the power switch and starts a new conduction phase.
FIG. 2 shows the scheme of a flyback SOPS produced and marketed by Sanken, in which the auxiliary winding AUX is used to power the control circuits of the converter. By way of a delay network Tdelay, switching of the comparator C2 is synchronized with a null voltage on the current terminals of the power switch to turn on the power switch. The power switch is turned on in a quasi-resonant condition to avoid a hard switching mode of operation of the converter. The delay network is commonly formed with external discrete components.
The SOPS control, intrinsically a variable frequency type of control, forces the flyback converter to function close to the limit between a discontinuous mode and a continuous mode. The discontinuous mode is the zeroing of the current in the primary during an off phase. In the primary winding, a current continues to flow during an off phase of the power switch. Zeroing of the current in the primary during an off phase is the limit of the discontinuous mode.
The secondary control loop includes an output voltage error amplifier ERROR AMPLIFIER, whose output is photocoupled via a photodiode and a phototransistor to the inputs of the comparators COMP1 and COMP2. The control loop also includes the RC OSC network and the OSC circuit which intervenes to control variations of the output voltage Vout. Variations of the output voltage Vout occurs as a function of the transfer of energy stored in the flyback transformer to the load. Therefore, the control loop is unable to handle the start-up phase, which is the charging transient of the capacitance coupled to the converter output.
This inability is commonly overcome by implementing a primary control loop by way of the comparator COMP1, the circuit ENABLING CIRCUITS and the logic AND gate. A fixed frequency oscillator OSC is used by the primary control loop to turn on the power switch P.omega. with the rising edge of a fixed frequency clock signal provided by the OSC oscillator. This permits the start-up of the flyback converter until it reaches an output voltage sufficiently high to cause a proper self-oscillating mode of operation.
During the start-up phase, the OSC oscillator imposes a fixed duration off phase (T.sub.OFF) according to a Pulse Ratio Control (PRC) mode of operation. Commonly, the OSC oscillator frequency is set by the RCOSC network to a value lower than the self-oscillating frequency of the flyback converter during its steady state functioning. The RCOSC network is made of external components. This ensures, at a steady state, a discontinuous mode of a self-oscillating converter (SOPS) which is intrinsically a very controllable mode of operation.
Normally, in a SOPS system, when the load applied to the output decreases, the switching frequency increases and this increments the losses due to switching of the power switch. Moreover, the unavoidable delays of the secondary control loop during the self-oscillating steady state functioning of the SOPS and the turn-off times typical of DMOS transistors dictate a minimum turn-on time (T.sub.ON) of the converter. DMOS transistors are commonly used as power switches. Therefore, a certain minimum quantity of energy is stored in the transformer during each turn-on phase. This energy is completely transferred to the load. If the load absorbs an amount of energy lower than the minimum, the output filter capacitor overcharges and, therefore, the output voltage Vout increases.
In these circumstances, the control system may only intervene by turning off the converter. This is done by disabling turning on of the P.omega. switch via dedicated ENABLING CIRCUITS until the Vout drops below a certain threshold. This determines a burst mode of functioning, during which the SOPS converter functions by alternating periods of normal functioning with periods in which the switch is off.
Due to inevitable delays and the turn-off time of the power transistor, SOPS are not suitable to supply relatively small loads and/or for prolonged stand-by conditions. For these applications, a control system capable of limiting the power transferred to the output in a more effective way, such as, for example, a fixed frequency control, is commonly preferred. Normally, when the load is reduced to stand-by conditions, a fixed frequency mode of control is enabled to facilitate the control and reduce losses.
The consequent decrease of the energy transferred to the output, and the limitation of the overshoot of the output voltage may reestablish the conditions for a transition to a variable frequency SOPS control mode which may cause a new overshoot of the output voltage. The converter may then oscillate between two modes of control--the variable frequency SOPS control and the fixed frequency control. The fixed frequency is at the frequency of the start-up and recovery oscillator.
Under these conditions, the current supplied by the error amplifier of the output voltage, and consequently, the current that flows in the power switch is a waveform that has a sequence of highs and lows. A similar behavior may be observed when the load of the converter is subject to an abrupt step-wise drop. Even in this case, the abrupt discontinuity of the load level induces an overshoot followed by a sinusoidal decay pattern of the current profiles of the error amplifier and the power switch. These behaviors, as well as a burst mode of operation, cause inefficiencies and a significant increase in the electromagnetic noise produced by the converter.